This invention relates to the control of fluid flow and, more particularly, to a digital fluid flow control system with a trim adjustment.
In a digital fluid flow control system, a plurality of individually actuatable, digital valve elements are interconnected in parallel between an upstream manifold and a downstream manifold. The digital valve elements are bistable, i.e., either open or closed, and are actuated by binary electrical signals weighted in value according to a binary code. The cross-sectional orifice areas of the valve elements are also weighted relative to each other according to the binary code. The fluid flow rate from the upstream manifold to the downstream manifold is controlled by opening and closing the valve elements in different combinations, responsive to the binary actuating signals; due to the identical weighting of the binary actuating signals and the orifice areas of the valve elements, the sum of the orifice areas of the open valve elements is proportional to the number represented by the binary actuating signals in the binary code.
The requirement that the cross-sectional orifice areas of the valve elements be weighted relative to each other according to the binary code imposes close tolerances upon the dimensions of the valve elements during manufacture. In order to calibrate the valve elements accurately to achieve the proper relative weighting, the valve elements must usually be removed from the valve body in the course of calibration one or more times for reboring or reworking. The closer the tolerances are held, the more reworking is required. The problem of proper calibration is particularly troublesome when the flow determining orifices are converging-diverging nozzles, as disclosed in a copending application of Harry Friedland and Addison W. Langill, Jr., Ser. No. 432,153, filed on even date herewith, and assigned to the assignee of the present application by an assignment recorded on even date herewith. Since the nozzle shape is important, a change in the effective cross-sectional orifice area of the nozzle requires complete reworking of the nozzle dimensions.